The invention relates generally to the field of CMOS image sensors and, more particularly, to such image sensors having transfer gates having regions of enhanced conductivity in the substrate as defined by the area of the transfer gate.
Referring to FIG. 1, prior art CMOS image sensors 10 are solid state imagers built in a substrate 5 wherein each pixel typically contains a photo-sensing region 20, a reset transistor 30, a charge transfer region 40, a transfer gate electrode 45 separated from the substrate 5 by an insulating layer 42, a charge to voltage conversion region 50, and an amplifier 60. In these CMOS sensors, dark current is generated in several regions of the device. Such prior art devices have addressed dark current reduction in the photo-sensing region 20. Another region of dark current generation is the charge transfer region 40.
With typical processing and design, and due to short-channel effects, the charge transfer region 40 of current day CMOS image sensors is basically the channel region of a xe2x80x9cnormally onxe2x80x9d, or depletion-mode type NMOSFET. Unlike CCD based imaging systems, typical power supplies used in these CMOS systems are unipolar, (e.g.,+3.3V). As such, the xe2x80x9coffxe2x80x9d state voltage on the transfer gate 45 of the CMOS sensor 10 is limited to a minimum of 0V (ground). Therefore, since the transfer gate""s surface is not xe2x80x9cturned offxe2x80x9d, a relatively high amount of dark current is generated in this region. This dark current, or a portion thereof, may leak into the photodiode during the integration period causing excess noise, thereby limiting the imaging performance of the device. To avoid this, it is desirable to provide a charge transfer region 40 whose surface region will be accumulated (with holes) in the off state (Vg=0V) so as to quench this excess dark-current generation.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a CMOS image sensor comprising (a) a substrate of a first conductivity type; (b) a photodctector for capturing incident light and converting it to a charge; (c) a transfer gate for passing the charge from the photodetector; and (d) a region of the first conductivity type of enhanced conductivity in the substrate which extends substantially along an entire length and width of the transfer gate.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
Advantageous Effect Of The Invention
The present invention has the following advantages of reducing dark current generated in the area of the transfer gate of a CMOS image sensor for reduced noise, increased dynamic range and overall improvement in image quality.